1. Field of the Invention
The present invention relates to an optical information recording medium capable of recording information by irradiation with a light beam, and more particularly to an optical information recording medium of a so-called sample servo type, in which prepits for detecting tracking signals are formed in a scattered manner along an information track.
2. Related Background Art
The optical memory industry expanding in recent years already encompasses various types of media such as optical video disks and compact disks for reproduction only and add-on, that is, direct read after write type optical disks utilizing thin metal films or coloring matter recording materials, and development is being made toward rewritable optical disks utilizing magneto-optical recording or phase transition recording. Also, the applications of such media are expanding from consumer uses to external memories of computers.
In such optical memories, particularly important are control technology for tracing a desired track with a small light spot, and record/reproducing technology for enabling high density recording and high speed signal reading.
In the field of control technology, there has been proposed a sample servo system, in which servo time and record/reproducing time are divided in time for excluding mutual crosstalk of signals.
FIG. 1 shows a part of a format of an optical disk of such a sample servo system. A block 50 is present in a number of 1,000 to 2,000 in a full turn of the disk. The block 50 is divided into a servo area 51 and a data area 52, and the servo area 51 is composed of a tracking area 53 containing wobbling pits 56, a focusing area 54 formed as a mirror surface, and a clocking area 55 containing a clock pit 57 for timing the data recording and reproduction.
The tracking operation is conducted according to the difference in the light amounts in reading the wobbling pits. Also, the focusing is achieved according to the change in reflected light amount from the mirror surface.
The sample servo method has the advantage of an absence of crosstalk or mutual interference of the signals, as the servo area and data area are mutually separated in time and space.
In the conventional optical disk employing such a sample servo method, the width of the recorded pits depends on that of the recording beam spot since the data area is formed as a mirror surface. As an example, the mode of recording on a magneto-optical disk is schematically shown in FIG. 2, wherein a magneto-optical disk 91 is composed of a transparent substrate 92 such as of acrylic resin or polycarbonate resin, and a magnetic recording layer 93 is formed thereon for example by sputtering. The information is recorded by irradiating the recording layer 93 with a light beam 105 focused through an objective lens 104 for heating the irradiated area, and simultaneously applying an external magnetic field by a device (not shown), thereby orienting the magnetization of the irradiated area into a desired direction. Thus, the information is recorded by the arrangement of magnetic domains (record pits) in which the direction of magnetization is different from that in other areas.
In the course of such recording, the temperature of the magneto-optical disk is highest, as shown in FIG. 2, at the center of the area irradiated with the light beam 105, and becomes gradually lower almost concentrically. Thus, a record pit of a width x is formed for the recording layer 93 with a Curie temperature of 200.degree. C.
In such information recording, the records on mutually adjacent information tracks may mutually overlap if the automatic tracking contains an aberration. Also, in a rewritable recording medium, the information previously recorded and not completely erased may undesirably affect the reproduction of newly recorded information. As an example, FIGS. 3A to 3C illustrate the record pits in overwriting of information by modulating the external magnetic field in the above-mentioned magneto-optical disk. FIG. 3A is a plan view of pits (hatched areas) recorded without tracking error with respect to the center C of the track. On the other hand, FIG. 3B shows an overwriting, on the pits (indicated by hatched areas) deviated upwards with respect to the center C of the track, of new pits deviated downwards as indicated by broken lines. Such overwriting operation provides pits as shown in FIG. 3C. Such an unerased portion of previously recorded pits significantly deteriorate the quality of reproduced signals.